Geology in space

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Winter in the northern hemisphere is synonymous with snow (at least in films and Victorian novels). The Earth is unusual because it has water can commonly be found on its surface in solid, liquid and gas form, and both solid and liquid water can rain/snow/sleet/drizzle from the sky. So what happens on other bodies in the solar system?

Clouds of ice crystals on Mars – Ready to snow? (NASA/JPL)

Most water on the surface of Mars is now trapped in its glaciers/ice caps, but what about snow? Whilst there is less water vapour in the Martian atmosphere than the Earth’s, Mars still has clouds made of ice crystals. These ice crystals can fall slowly out of the sky, but at night under the right conditions, snowstorms can occur. Most of this snow will vaporise before it hits the ground, and any that do make it to the ground will likely vaporise during the day.

Maxwell Montes the highest mountain on Venus, appears brighter than the surroundings because of sulphide frost (Image NASA/JPL)

Venus has the hottest surface temperature of any planet in the solar system. Water snow is not possible. The atmosphere of Venus contains lots of sulphur and temperatures hot enough to melt lead, it is so thick we can’t see the surface of the planet so we have collected radar data to understand what it looks like. One feature of this data is that on the tops of some high mountains there is an increase in the number of radio waves reflected, giving the appearance that the tops of mountains are brighter, just as snow on the top of mountains is brighter than the surrounding rock.

Whilst it is far too hot for snow on Venus, it is thought that these mountain tops are covered in a form of frost caused by metal sulphides, which forms only at high elevations and it’s this which makes is appear more reflective to radar.

Mercury, my main planet of study, doesn’t have snow-capped peaks, being too hot over the majority of the planet and no atmosphere. So the nearest feature to snow is ice in the craters close to the north and south poles, these craters are permanently in shadow protecting the ice from the boiling Sun. Without an atmosphere there isn’t any form of precipitation other than a daily micrometeorite shower.

The south pole of Mercury, the black areas are permanently in shadow and thought to contain water ice. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Titan, a moon of Saturn is much to far out from the Sun to have water snow, and any water near the surface, just like in the bleak midwinter, is like a stone. It does snow on Titan, near the poles hydrocarbons can crystallize out in the atmosphere and then fall as snow down onto the surface.

Another moon of Saturn, Enceladus, is probably the most visually spectacular example, geysers on the south pole can be seen erupting water vapour into space. Some of this water is lost in orbit and forming the E ring of Saturn, but some of the water falls back down onto the surface as ice crystals. As this fall back it is happening without an atmosphere it doesn’t really count as snow but is a great excuse to show the stunning photos of the plumes.

Often during Halloween ghost stories are told. Phantoms from the past linger in the present trying to tell us about some previous horror. We know that there are ghosts on Mercury.

Mercury is small, smaller than the moons Titan and Ganymede. Small bodies lose heat quickly and don’t generate as much internal heat as larger planets. That that is not to say that it was never hot, in the past Mercury would have been much hotter, and with that volcanically active. The planet’s surface has been covered over and reworked by volcanic activity often multiple times.

We see evidence of volcanoes on Mercury from Lava filled craters, and lava flows flowing out of them, bright spots on the surface which represent fire fountains and pyroclastic deposits. Smooth plains cover over much older crater marked surfaces.

Red spots in the middle of Derain crater, the crater has been filled in with lava giving it a smooth appearance, the red marking represent later pyroclastic eruptions (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Molten rock will follow weaknesses in the rock in order to find the easiest route, on Mercury, these are often found in impact craters, filling them up like bowls. As the bowls of lava cool, they contract the top starts to wrinkle, like the skin on custard. These wrinkles form long bands of hills imaginatively called wrinkle ridges.

In some places lava flows are very extensive forming planes which completely cover craters, concealing the scar left by an impact on the planet’s surface. because the lava is thinner at the rim of the crater and so cools and shrinks at different amounts to the lava surrounding this. The differential shrinking leads to the formation of either wrinkles or in some cases faults forming. The crater, completely buried by lava, becomes apparent, traced out on the surfacece of the lava. The outlines in the lava are known as ghost craters.

Since graduating from my masters I, have been working for a mining company in West Africa. I have had some great times meeting amazing people as well as seeing some amazing geology, at night, during in the dry season, I would get some awe-inspiring views of the rest of the universe.

My photo of Venus and the Milkyway take from Liberia in 2016

From an early age I was always fascinated with dinosaurs and volcanoes, so I tried to find out more about them, a terrible memory for names meant that the dinosaurs fell by the wayside, but my interest in volcanoes grew into wanting to understand how the Earth worked and slowly I turned into a geologist.

A rocky shoreline on Titan ESA/NASA/JPL/University of Arizona

At the same time, the rest of the universe has been there from staring at shooting stars to the Galileo mission, it’s photos of Jupiter and the moons were part of my childhood. In 2005 the photos Huygens probe touchdown on Titan opened my eyes to how surprisingly familiar alien worlds can be (maybe that says something about British seaside holidays).

So after 6 years as a geologist on Earth, I realised one planet was not enough, it’s time for a change of course. I have just started PhD at the Open University where I will be researching Mercury using Nasa’s MESSENGER data to make geological maps of part of the planet, contributing to our knowledge of the smallest planet in the solar system. During the coming weeks, I hope to publish some more specific blogs on Mercury as well as the more general planetary geology and from time to time I’ll post updates on my progress.

Old river sediments in Brazil and the Central African Republic contain minerals found nowhere else on Earth; a form of diamond, dark black or green-gray, mixed with a lot of graphite (the stuff inside pencils) and often with small bubbles inside.

These diamonds are unusual for a variety of reasons, they make up some of the largest diamonds ever found and yet the origin of them is unknown. Most diamonds come from kimberlites; volcanic tubes which shot diamonds up from the Earth’s mantle up to near the surface (in some areas diamonds are mined from kimberlites, in others, they are found in rivers downstream of them). Carbonado diamonds don’t have any signs of kimberlites associated with them they also don’t include any material from the Earth’s mantle, which can often be found trapped inside diamonds.

Diamond is formed at high pressure and temperatures and so doesn’t often have any pores or bubbles within them, but these types of diamonds are often very porous (full of holes). The type of carbon which makes them up is also unusual. Whilst all diamonds are made of carbon, carbonados are made of a lighter form (isotope) of carbon than other diamonds found on Earth. Dating has shown them to be at least 3.8 billion years old, half a billion years older than the oldest known kimberlites adding to the mystery of their source.

Their two geographical locations can be easily explained by plate tectonics, whilst currently, separate the two were at one stage part of the same continent. (Later geological processes affecting more the countries to the west of CAR in Africa). This point source location, unusual isotopic composition, and the high-pressure high-temperature but also high gas pressure (allowing all the bubbles to form). Narrow down the possibilities of how these diamonds formed and suggest an extraterrestrial origin.

The remnants of a supernova – any diamonds here? (NASA/ESA/JHU/R.Sankrit & W.Blair)

The most exciting hypothesis is that these diamonds formed in a supernova; the exploding death of a star, this would explain both the carbon isotopes and also the bubbles. So whilst Thor’s hammer might have been forged from a neutron star, these diamonds found on Earth may come from the remains of an exploding star. Other theories include being knocked off a white dwarf star or coming from one of the giant planets in the solar system or even from an exoplanet. This is an area of active research and further understanding of the processes going on in these high-pressure temperature environments will help narrow down the candidates but it’s amazing to think that these diamonds may have come from outside our solar system.

There are a lot of upcoming space missions to be excited about; BepiColombo’s trip to Mercury, ExoMars, the Europa Clipper and New Horizons second flyby to name a few. One that has me really excited is Psyche.

Psyche is a NASA mission to explore the asteroid Psyche 16. This is an M-Class asteroid, which means that it is made dominantly of metals, mostly Nickle and Iron, compared most others types of asteroids which contain much more silicates (rock) within them. This unusual composition suggests that it is a relic left over from the formation of the planets.

Early in the life of the solar system, a protoplanetary disk of dust began to form clumps of rock as they collided, gradually these lumps of rocks grew into objects known as planetesimals.

When these planetesimals became large enough, the internal heat, generated by a mixture of gravitational energy, radioactive decay, and heat generated by collisions allowed those larger than about 30 km to internally separate out like oil and water; the denser materials sank to the bottom whilst the lighter material floated to the top. Iron, nickel, and a range of other metals sank to the bottom, it is this same process which formed the Earth’s core.

We have never sampled the Earth’s core, only being able to use seismic, magnetic, gravity and density data to infer information about it, as well as studying the limited number of meteorites which may be remnants of a core exposed by collisions with other planetesimals.

Psyche is the only known exposed planetesimal core in the solar system. This makes it a truly unique and exciting place to visit as it will enable us to look directly at and take measurements on the same kind of material which makes the core of the rocky planets. This will be the first time we have visited a metal body and is likely to look unlike any other body is so far seen in the solar system, possibly with impact craters showing metallic jagged edges.

Psyche above Psych SSL/Peter Rubin

The Psyche mission is scheduled to launch in 2022 and reach the asteroid in 2026, it will be exciting to see what new insights about the formation of the solar system and our own core.

Some of the most spectacular images from within our Solar System come from Nasa’s Cassini spacecraft as it orbits Saturn. One of the reasons for this is that many of the vistas are dominated by the planet’s rings.

Saturn, its rings (and their shadow), and the moon Tethys (NASA/JPL/SSI)

The rings of Saturn are made up mainly of lumps of ice (with a small amount of dust), which while individually are only millimeters up to about a kilometer in size, add up to whole rings structures which whilst only a kilometer deep extend laterally for hundreds of kilometers.

There is still a lot of debate about how they formed and how long they will last. Cassini’s final orbits are designed to answer some of these questions by flying between the planet and the rings and we should get more answers in the months and years after the end of the mission once the data is analyzed. Current theories suggest that the rings formed around the same time as Saturn did; either from the same planetary nebula which formed the planet or from a moon which was torn apart by Saturn’s gravity. Other researchers have suggested that they are only 100 million years old and transient features which will eventually fall into the planet (over very long time periods). whilst the majority of the evidence points to them being very long lived so far. The exception to this is the outer E rings, these are known to have been generated by the ice volcanoes of Enceladus throwing out icy material into the orbit of Saturn and form a ring.

Rings are dynamic with transient features such as “spokes” of dust caught in the magnetic field chasing around jus above the surface of the rings, disturbances within them like propellers formed by the gravitational distortion of moonlets, or the ripples generated by Daphnis as it travels within a gap in the rings.

Daphnis a small moon causing ripples within the rings of Saturn (NASA/JPL/SSI)

Pan, a moonlet within a gap in the rings has made the gap by collecting up the ice particles, forming a band around its middle.

Pan, which orbits within the rings, has built up ice within its middle (NASA/JPL/SSI)

It is not just Saturn which has rings, Neptune, Uranus, and Jupiter all have rings and even the 250 km diameter small planetoid 10199 Chariklo has a ring system around it. Temporary rings probably occur on all planets as comets and asteroids get too close are torn apart by the gravitational pull. forming a ring of debris which would eventually fall to the surface, Mars may develop a transient ring system, when Phobos is torn apart by tidal forces as it slowly spirals towards the surface. This ring will be transient as the material itself falls onto the surface of Mars.

Infrared image of the rings of Jupiter

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Since Galileo first pointed a telescope at the Moon and saw its marked surface people have been debating how the craters they observed formed, for much of history there were two interpretations; either an impact crater or formed by volcanic eruptions.

Impact or Volcano Picard crater -Luna Oribter 4 (NASA)

It was not until the space age when close up photos (and later moon landings) revealed most of the creators on the surface were formed by impacts (volcanoes do exist on the moon but form very different features). But what are the differences and how can you tell remotely without landing?

Barringer Crater Arizona, formed by a meteorite impact (USGS)

On first glance, both types of craters can look very similar, both would form round features with rims. Both can have ejecta around them, formed from the material thrown out from the inside (either the material the surface is made from or ash from volcanic eruptions).

Aniakchak, Alaska volcanic caldera (US National Parks service)

However when you think about how they formed you can understand some of the differences when looking at them: volcanoes are explosive, expansions of material often with repeated events close to or in the same place.

Most volcanoes are built upon the landscape and so often the inside of the crater will be at a higher altitude than the surrounding terrain (with the exception of Maars due to interactions with water under the ground and features called calderas which are collapsed volcanoes). Whereas impact craters are formed by a surface beeing excavated by an impactor, whilst the sides of the crater can be built up from material thrown out of the crater floor itself will be lower than the surrounding terrain.

Volcanoes often show evidence of their volcanic nature around them, whether it is lava or ash flows inside and on the sides of the volcano. The volcano itself will be built up of volcanic rock. An impact craters wall will be made up of the same material excavated from the site. there are of course exceptions to this, for example on Mercury some of the impact craters are filled in with lava which broke through the bottom of the crater, however, the wall of the crater is of different material to later lavas which fill in the basin.

Volcanic craters can also be asymmetrically distorted by the wind or the direction of the eruption (or deformed by later eruption events). The overwhelming majority of impact craters are circular. Larger craters also have a central mound formed by the rebounding material which you don’t see in volcanoes (though new volcanic cones can grow in existing craters).

There are lots more features of craters evident when you get a close-up view on the ground but they are hard to observe from satellites where most of our planetary data comes from. As with much of geology, by observing a combination of these features a story of where a crater came from and how it formed can be built up from remote sensing data.